Current challenges of cancer research focus on understanding the stem cell-like properties of a cancer cell, which many believe are responsible for drug resistance, recurrence and metastasis. We lack definite markers of therapeutic value in cancers of heterogeneous nature such as breast cancer. In mouse models of breast cancer a marker of cancer stem cells Stem cell antigen-1 (Sca-1) has been associated with drug resistance and metastasis. Sca-1 has been shown to enhance disease progression, emphasizing the importance of cancer stem cells in tumorigenesis. My recent work has provided significant insight into Sca-1 function, demonstrating that Sca-1 binds to TGF receptor 1 to disrupt the TGF- receptor complex and Smad3 signaling leading to tumor progression. I also found that Sca-1 could activate PI3K/AKT/ERK signaling and inactivate PTEN in mouse mammary tumors. Human breast cancers show disruption of TGF- signaling and activation of PI3K pathway but they lack murine Sca-1 gene. I set out to see if human homologues of Sca-1, a family of Ly6 genes regulate TGF- and PI3K pathways and if they are relevant to human breast cancer or cancer stem cells. Oncomine data analysis on more than 600 primary invasive ductal carcinomas (TCGA, NCI), confirms a gain in copy number and increase in gene expression of Ly6E and Ly6K in invasive ductal carcinomas in comparison to normal breast tissue. My current data indicates that Ly6E is overexpressed in estrogen receptor alpha (ER) positive, antiestrogen sensitive breast cancer cells and it activate PI3K pathway. In contrast, I found that Ly6K is over expressed in ER negative, antiestrogen resistant cancer cells and it disrupts TGF- signaling and activate PI3K pathway. This suggests that the signal transduction and growth-promoting phenotype of Sca-1 is passed on to its human isoforms Ly6E and Ly6K during evolution in gene specific manner. This proposal will test my hypothesis that Ly6K and Ly6E regulate cancer stem cells in ER negative and ER positive tumors and regulate TGF-, PI3K, and related signaling nodes, thereby altering the phenotype and drug responsiveness of resultant tumors. The outcome of this proposal will significantly advance our knowledge of mechanisms of tumor progression and drug resistance in breast cancer and present us with new therapeutic targets and biomarkers.